The treatment of food, waste, drugs and medical products to inactivate noxious biological organisms is currently being done by thermal, chemical and ionizing radiation processes. For example, the use of chlorine to treat water creates chlorine-related chemicals which may be carcinogenic and which often causes eye irritations in swimming pools. The use of heat to sterilize or pasteurize food often changes the taste and flavor. Similarly, in the case of nuclear radiation, the effects of ionizing radiation are such that it changes the chemical composition and thereby the taste of the treated food. Medfly Infestation in oranges or in other fruit is also a problem, currently dealt with by the use of toxic gases.
The use of electric fields to destroy noxious biological organisms is a possible alternative. The principal benefit, if properly applied, would be the ridding of the material of the noxious organisms, while at the same time not significantly raising the temperature, incurring by-products of ionizing radiation, or causing the development of new chemicals of unknown effects.
Early investigators considered the use of radio frequency (1 MHz to 600 MHz) or microwave frequency (1 GHz to 10 GHz) electric fields to provide non-thermal electric field destruction of noxious organisms. However, past efforts employing these frequencies have not caught on. This is due, in part, to inappropriate equipment designs or poor biological science, or a combination of both. Where science has been rigorously adhered to, little or no disinfection or sterilization by radio frequency (RF) or microwave frequency has been observed. Part of the problem is that most of the research in these frequency regions has been done on the basis of available equipment, and not on the basis of a first principal which exploits "an undisputed kill mechanism". Furthermore, almost all test procedures and protocols which claim significant positive effects often resulted in considerable localized heating, thereby confusing thermal effects with the purely athermal electromagnetic effects.
For example, many attempts have been made to use electromagnetic energy, principally in the microwave or radio frequency range, to effect pasteurization, disinfection, or sterilization. A good summary of this past work appears in a book by Robert V. Decareau entitled "Microwaves in the Food Processing Industry", published by Academic Press, 1985. In this summary the conclusion is that the use of shortwave band or higher frequencies for the abovementioned purposes either does not occur or is not sufficiently reliable for food processing or other disinfection purposes.
Apart from the radio frequency and microwave regime, the bactericidal and possibly insecticidal effects of electric currents have been investigated by simply passing direct current or low frequency alternating current through the host product. Other applications considered the passage of low frequency current to create ohmic heating throughout the volume of the medium under treatment. Other methods have employed the use of high voltage arc discharges to create hydraulic shock waves.
Recently, and quite apart from the use of electric fields to rid host material of noxious biological organisms, several separate lines of investigation have clearly demonstrated non-thermal or "sure-kill" electric field effects. These studies are identified in the disclosure statement accompanying this application.
As such, the known "undisputed kill mechanisms" generally involve the application of a sufficiently large amplitude unipolar electric field intensity over an appropriate duration to biological organisms (such as bacteria) to irreversibly rupture cell wall membranes in the organisms. Experimental data demonstrates that this can be done by application of electric field intensities in the order of at least a few thousand to ten thousand volts per centimeter for the duration of at least 0.2 microseconds and preferably longer. Such enormous field intensities are very difficult to generate and apply by conventional equipment without arc-over or electrical breakdown. If not optimized, high power consumption could well result in temperature increases in the material being treated.
Thus, the "sure kill mechanism" depends on the application of a unipolar pulse of sufficient duration and amplitude to cause breakdown of the cell membranes. Such unipolar pulses are needed to cause a charge to build up across the membranes and when maintained at a sufficient amplitude, about 3000 v/cm or greater, and adequate duration, in the order of 0.2 microseconds and greater, will eventually cause the membrane to rupture or otherwise be disabled.
One investigation, for example, was a study to discover ways in which to improve the treatment of electrical burns experienced by power-line workers. In this study it was reported that two phenomena take place during electrical shock (assuming that fibrillation does not occur). These phenomena are cell degradation by the Joule heating and, second, by non-thermal cell-wall rupture effects of strong electric fields. In a study by Diane C. Gaylot ("Physical Mechanism of Cellular Injury in Electrical Trauma", Dr. of Philosophy dissertation at the Massachusetts Institute of Technology in September of 1989), she demonstrated that cell membrane disruption by electrical stress occurs and can lead to irreversible effects and ultimate destruction of the cells. She presents this as over a dozen references regarding the effects of intense electric fields which lead to irreversible breakdown of the cell.
Other investigators have used, as a standard laboratory technique, brief and yet intense electrical pulses that produce cell fusion or change genetic material. A review of this work with extensive references is given by Ulrich Zimmerman in "Electrical Breakdown Electropermeabilization and Electrofusion", Rev. Physiol. Blochem. Pharmacol., 105:176-256. A variety of electrode configurations are employed with the principal objective to effect reversible breakdown for cell fusion and genetic research.
Recently, in Dunn U.S. Pat. Nos. 4,695,472 and 4,838,154, methods and apparatus are described which utilize the membrane rupture effect to aid in the preservation of food products, particularly liquid food products such as milk, fruit juices and fluidized eggs. While apparatus such as described by Dunn is suitable for treating liquid foodstuffs in limited volumes, the method and apparatus is not suitable for the treatment of large volumes of material contaminated by noxious biological organisms because it may require unduly large amounts of energy, thereby rendering the process uneconomical. Furthermore, the electric field applicators described by Dunn are far from optimum and are prone to electrical breakdown, caused by the false assumption by Dunn that a uniform electric field represents the optimum way to present electrical breakdown. In the processes described by Dunn, uniform electric fields are created by parallel plates. However, during the application of very high intense electric fields to materials between the plates, some type of heterogeneity such as a gas bubble may occur which will create a region of excess electric field intensity or microplasma which rapidly propagates, via electron avalanching plasma effects, from one plate to the other, thereby causing a highly conducting plasma-like streamer which shorts out the two plates and the effect of the electric field, thereby negating the treatment process. As will be shown later, the use of such parallel plates to create uniform electric fields actually tends to defeat the purpose of the system and can lead to system instability unless the material being treated is ideally homogeneous.
Doevenspeck, in U.S. Pat. No. 3,265,605, shows a basic configuration which is less likely to develop streamers except in one region. This configuration consists of a short carbon rod coaxially located in a longer hollow metallic or carbon cylinder. However, excessively large electric field intensities are created in the region near the edges or exposed end of the solid cylinder or electrode in the treatment region, thereby forming microplasmas which can initiate streamers or arc-overs. Furthermore, Doevenspeck does not address any type of optimization procedures and does not specify pulse widths or other parameters which lead to optimum and economical operation.
In U.S. Pat. No. 4,457,221, Geren describes sterilization apparatus designed for killing organisms in place within the host by means of successive, relatively long duration, high current density pulses of electricity of alternate polarity. The pulses are passed through the host and contaminating organisms for a period of a few seconds. The current is conveyed to the host from electrodes immersed, with the host, in a weak electrolyte. Geren proposes to use a pulse duration of 200 microseconds to 5 milliseconds, of successive alternate polarity with a current range of 50 milliamperes to 5 amperes per square centimeter, at a pulse rate between 100 and 1,000 times per second. In this instance, 200 microsecond pulses, repeated 200 times per second, are too long and too energy consuming in most practical situations involving high conductivity materials. Furthermore, the 5 amperes per square centimeter current density may not be sufficient to achieve the required sure kill electric field intensity at room temperatures. Methods to optimize the conductivity values for the "weak electrolyte" are not presented.
In U.S. Pat. No. 3,095,359, Heller proposes employing a frequency from 1 to 250 megacycles at voltages of up to 100 kilovolts with pulse widths ranging from 1 microsecond to 10 milliseconds at pulse repetition rates of 30 to 10,000 pulses per second. He also proposes exposing the microorganisms in a medium such as water, which is isolated from the electrodes via a thick sheet of dielectric material, such as plastic. However, the use of such plastic isolators, even though very thin, increases the source voltage requirements to impractical levels. Furthermore, Heller proposes to use sinusoidal waveforms which reverse the polarity, thereby reversing the charge distribution and negating the charge buildup across the membranes.
L. I. Markitanova, as reported in the Journal of Applied Chemistry of the U.S.S.R., Volume 59 (11)/pg 2, 1986, reports on the destruction of bacteria in treated waste waters by the application of direct current voltages across the cells. The voltage applied was of the order of 200 volts per centimeter and the dwell time was of the order of 25 seconds. Although bacteria were reported destroyed, this approach could result in a substantial temperature rise as well as inefficient use of electrical energy, especially for low resistivity liquids, as well as electrode erosion.
Thus, one of the keys to success in the application of the "sure kill mechanism" is to use either very high intensity unipolar pulses or sinusoidal waveforms of very low frequency, such that an electrical charge can build up and can be maintained and sustained over a sufficient period of time, thereby causing rupture of the membrane. A second key is to use pulses of the smallest possible duration to minimize energy consumption. The third and most important key is the use of an exposure system which is essentially immune to electrical breakdown effects.
To overcome the foregoing difficulties, this invention describes a method wherein the power consumption can be optimized by reducing the pulse width. Such a reduction is realized at the expense of increasing the amplitude of the applied pulses. However, such a pulse-width energy reduction process can be continued until the "sure kill" energy consumption requirements of the pulse abruptly increase. Such an increase occurs at approximately one microsecond, depending on the type of biological organism or entity to be destroyed. The requirement for increased field intensity leads to a second requirements that the field exciter must be highly immune to the generation of streamers or sparke discharges. As opposed to parallel plates, which create a uniform field, it can be demonstrated that a nonuniform field is more appropriate, a field distribution which inherently suppresses the development of short circuits or streamers. The optimum arrangement requires that the current density in the medium being treated be controlled, rather than the field intensity, and that the optimum electric field distribution is nonuniform rather than uniform. Also, the charge densities or Current densities on the electrodes or in the material being treated should not vary abruptly or vary greatly; those densities should not change significantly even though the dielectric properties of the media being treated may change, either locally or generally.
The above is analogous to supplying electric energy from either a constant voltage source or a constant current source to a resistive load whose resistance decreases with temperature as the load resistor heats up. If a constant voltage is applied to such a load, the tendency is for the load current to increase progressively, thereby increasing the dissipation of heat because, as the resistance of the load progressively decreases, the power dissipated increases as the product of the applied voltage and current. If the heat liberated in the load resistor is not rapidly removed, thermal runaway can occur, which leads to the destruction of the load resistor. On the other hand, if the load resistor is energized by a constant current source, any decrease in the load resistance due to temperature increase results in a lower dissipation because the power dissipation is proportional to the product of the resistance times the square of the current. Similarly, in the case of the controlled current density method, any decrease in the resistivity in any local volume of the material being treated due to a heterogeneity results in both reduced power dissipation and reduced electric field intensity. The local electric field intensity is controlled by the product of the current density (which is largely constant) and the resistivity (which decreases upon the onset of a streamer or arc) . Thus, a controlled or constant current density applicator tends to snuff out incipient arcs at the outset. On the other hand, a uniform electric field applicator tends to induce a runaway streamer or arc, since the dissipated power progressively increases once an incipient arc is formed.
To treat solid material, it is necessary to encase the host material, particularly if it is all solid, by immersing it in a dielectric of similar value, such as in water or foam or some other type of fluid medium. If the material being treated is a relatively good or low loss dielectric, equipment to recover the energy stored in the applied field is also a necessary requirement to achieve optimum efficiency. Post-treatment techniques may also be needed, either to recycle the thermal energy in the treated material (if pre-heated) or to recycle various types of encasing dielectrics such as deionized water or conducting foam. Means to sense the electrical properties of the fluid media are necessary in order to apply the proper current densities.